function objTOsearchStr(obj){
    let str = '';
    for(let key in obj){
        str += `${key}=${obj[key]}&`;
    }
    return str.slice(0,str.length - 1);
}

function ajax(method,url,params){
    return new Promise((resolve,reject)=>{
        let xhr = new XMLHttpRequest();
        xhr.open(method,method == 'get' ? url + '?' + objToSearchStr(params) : url);
        xhr.onreadystatechange = function(){
            if(xhr.readyState == 4){
                resolve(JSON.parse(xhr.responseText));
            }
        }
        xhr.setRequestHeader('content-type','application/x-www-form-urlencoded');
        xhr.send(method == 'get' ? null : objTOsearchStr(params));
    })
}